The present invention relates to jigs for separating ore concentrate from pulp material and more particularly to such jigs utilized to separate a heavy fraction and a lightweight fraction.
The mineral jig, as defined by Arthur F. Taggert in his book entitled "Handbook of Mineral Dressing, Ores and Industrial Minerals" published by John Wiley & Sons, Inc. (4th Printing, September, 1950), is a "mechanical concentrator that effects a separation of heavy grains from light by utilizing differences in the abilities of the grains to penetrate a semi-stationary bed". Essentially it is a box with a perforate bottom and no top in which a relatively short range separating bed is formed by pulsating water currents. The "separating bed" is expanded when loosened by the water pulsation to form a "fluidic bed". The material may include "ragging" comprised of loosely collected particles of a selected size resting on a sieve.
The pulp material is passed over the ragging and sieve and therefore becomes an integral part of the "jig bed". Water pulsations are directed upwardly through the sieve and remainder of the jig bed to periodically create a fluidic bed with the pulp material. The lightweight fraction of the pulp will be pulsed upward through the fluidic bed while heavier fractions may move upwardly to a substantially lower elevation. After the pulsation, the lightweight fraction tends to settle slightly while the heavier specific gravity fraction settles more quickly through the intersticies between adjacent pulp material and the ragging (if used). After a series of pulsations, the heavier fraction will settle to the bottom of the bed while the lighter fraction or "tailings" move to the top and are shifted away from the bed by more incoming pulp material.
The mineral jig, as used today, is essentially a combination of two types of gravity separation systems, namely the rising current classifier and the heavy media separator. In order to understand how the mineral jig operates effectively, both systems must be addressed simultaneously.
The passage of fluid upward, or in opposition to settling forces acting upon the feed, acts to hinder or prevent the settling of particles from the feed. It is a fact that during classification within a rising current classifier, particles of varying density will settle at unequal rates. In applying this to a mineral pulp, small particles of heavy material would settle with larger particles of a lighter material, thus effectively preventing a close concentration of the heavy particles.
In a heavy media or sink-float system, separation of particles is achieved due to the differences in specific gravities of the particles. Within certain parameters the size of the particles has no effect on the separation, for a particle of one specific gravity or density which is less dense than the media will not settle at all but will remain at the surface of the media while the particle which is of greater density will settle through the media.
In a mineral jig, the upper layers of material represent the settling of particles as in a rising current classifier while the lower layers (ragging), being sustained in a fluid condition by proper pulsation, represent the action of a dense media. Thus it will be seen that large, less dense particles of material will settle through the upper layers of the jig bed to eventually encounter the lower heavier media layers where they will be rejected at that level.
This action cannot be detailed except in theory. However, it has been found that if proper layering of the jig bed cannot be attained then the concentration efficiency will be reduced or the machine may not operate effectively. The formation of effective layers of material within the jig bed is therefore of prime importance to successful operation of the jig.
Great effort and ingenuity have been expended to produce jig beds that will maintain the preferred layering along at least theoretically defined planes that are normal to the direction of pulsation. One method to accomplish this is to provide a perfectly horizontal sieve upon which to form the jig bed and providing a continuous, even feed to the bed. Such beds must be held rigidly in the horizontal orientation, otherwise the bed material will shift in one direction or another causing build-up in one area of the bed and decreasing the layer thickness in another area where the fluid pulsations will break through. The result is "boiling" in the jig bed which, in turn, upsets the efficiency of the entire bed for effectively separating the feed as desired.
Horizontal bed jigs are in current use worldwide. They typically are used with water pulsations which may be pulsed upwardly, downwardly, or both upwardly and downwardly alternately. Alternatively, the bed itself is "jigged" to produce the pulsing effect. The upward pulsation causes the fluidic bed to form and gravity acts to move the high specific gravity particles downwardly in the bed to form a concentration of the selected ore. Theoretically, gravity may be augmented by a reverse suction stroke following a positive pulsation. The suction stroke, conceivably, operates against the pulp material to increase the settling rate of the heavier, more dense particles. However, the suction produced by the pulsating water acts not only upon the heavy particles but also on the ragging and lightweight fraction as well. Often, the result is a packing of the jig bed that cannot be loosened by successive positive pulsations. Therefore, jigs that utilize only positive pulsations of water often prove more effective than those using both positive and negative (suction) pulsations.
The absolute requirement of formation of a uniform fluidic bed within a jig has dictated the horizontal, flat shape of conventional jigs. Even so, attempts have been made to increase gravity separation rate by rotating the jig bed to add a radial centrifugal force component greater than the pull of gravity to the settling particles. Attempts at this have been frustrated in the past mainly because the jig bed would not remain even. At best, such apparatus function as classifiers, and are not at all effective as concentrators.
U.S. Pat. No. 4,056,464 granted to D. J. Cross on Nov. 1, 1977, discloses a "jig" that utilizes a frusto-conical rotating bed for receiving pulp material. A slurry is placed on the rotor and is subjected to pulsations about its periphery. It is claimed that heavy material will pass through the screen (comprising the frusto-conical configuration) to be concentrated prior to collection while lighter material will move up the "bed" and spill over the edge for separate collection. The difficulty is that the rotating, frusto-conical screen produces outward centrifugal forces that are unequal along the rotating axis. For example, a particle near the vortex of the frusto-cone will have an outward, centrifugal force of, say, two times the force of gravity or 2 G's. A particle near the enlarged base of the frusto-cone having the same mass would experience a much heavier centrifugal G loading. Inward pulsations required to create a fluidic bed must oppose the centrifugal force acting upon the particles, causing them to settle. Therefore, a "fluidizing" force acting upon the particle rotating at a greater radius from the axis of rotation would necessarily be greater than that required to fluidize or lift a particle at a smaller radius from the rotating axis. The bed would therefore become uneven if it weren't held in place by the disclosed baskets. With the ragging material thus held in place, the device acts as a rotating screen, collecting some concentrate but primarily classifying the feed material.
Cross discloses in FIG. 7 of his drawings an upper cylindrical portion of the rotating frusto-cone. This area of the "jig", since it is at the maximum diameter of the frusto-cone, is inundated with pulp material that has been passed at increasing velocity over the conical sides. If the cylindrical area of the bed is to be fluidized, the force of the pulsation required to lift and open the bed at the maximum radius from the axis of rotation would be substantially greater than that required to fluidize the remainder of the "bed" held by the frusto-conical screen below. Therefore, the fluidic bed at the cylindrical screen cannot be properly fed due to the turbulence of boiling material caused by the pulsations against the feed along the frusto-conical sides.
It therefore remains desirable to obtain some form of apparatus that will effectively and at a relatively high rate of speed, separate fine heavy particles from lightweight particles in a pulp material.
The present invention was conceived to solve the problem by producing a cylindrical true "jig bed" that will maintain an even, layered bed while being rotated about a fixed axis. With such a bed formation, and with positive pulsation, increased settling rates may be achieved through the increased settling forces applied to the particles by centrifugal force.